It has now been appreciated that the general principle set forth in the prior application identified above has usefulness going well beyond the molding of hollow, slender parts.
For example, there are many instances in the manufacturing industry where it is desirable to provide a metallic or other stiff insert either entirely or partly within a molded plastic part, and in which the plastic part and the insert may both be relatively elongated. Particularly where the insert is to be entirely buried within the plastic part, the plastics industry has not discovered any reliable method by which to accurately position the insert within the part. As already pointed out, and especially in the case of elongated parts and elongated inserts, there is a strong tendancy during the injection molding process for the injected and highly pressurized plastic to push the insert to one side or the other of the cavity in which the part is to be molded, thereby causing the final part to be rejected, in many cases. In the case where the insert extends partly out of the molded plastic, there is the possibility of cantilevering the insert into the mold cavity in which the part is to be molded. However, here again, especially in the case of particularly long and slender cantilevered inserts, the pressurized plastic during its injection will tend to push the cantilevered end of the insert to one side or the other, thus preventing it from being centrally located in the final item.
A case in point relates to the manufacture of "wings" for eyeglasses. Most eyeglasses currently in use have plastic wings which are reinforced with a steel wire passing down the centre. The steel wire terminates in the hinge portion which connects the wing to the eyeglass frame, and most of the wire is inside the plastic. The present method for producting these wings involves firstly molding the plastic wings without any insert, and without any curvature. Then, an operator manually pushes a previously heated wire length-wise into the straight plastic piece. This operation requires a considerable amount of skill, in order to prevent the wire from straying from the centre of the wing. The incidence of scrap is very high. After the hot wire has been inserted, the wing is then bent or curved to its desired final shape.
The foregoing is only one problem among many encountered in the plastics industry, where it is required that a central insert be provided within a elongate part.